The present invention relates to a method of fault identification in a plasma processing reactor powered by an RF source.
Many thin film processes use plasma processes to facilitate the rapid and accurate fabrication of minute structures with desired properties. Plasma processes include the deposition and etching of metal, conductors and semiconductors on a substrate, for example, a silicon wafer. The plasma process usually involves placing the substrate in a vacuum chamber, introducing process gases and applying radio-frequency (RF) power, typically 0.1 to 100 MHz, to create a plasma. The plasma consists of ions, electrons, radical gas species and neutral gas, all of which permit the desired reaction to proceed. The plasma reaction has many inputs, including RF power, gas flows, chamber pressure, substrate and wall temperatures, chamber wall conditions, electrode spacing and so on.
Control of the complex plasma process is the key to improved manufacturing, i.e. to have accurate and repeatable processing the plasma itself should be repeatable. Unfortunately there are few schemes in existence for direct plasma monitoring and control. It is more usual to monitor or control gas flow, power output from RF generator, chamber pressure or temperature, etc., i.e. the process parameters or process inputs. However, since the plasma process depends directly on the plasma parameters, measuring these indirect variables is generally not sufficient. The plasma represents a non-linear complex load in electrical terms. This results-in the generation of harmonics of the RF driving signal. These harmonics, known as Fourier components, are very sensitive to changes both in the plasma process and the process parameters.
It is generally accepted that monitoring the Fourier components of the RF power signal provides a useful way to monitor the plasma process. These components are a more direct measurement of the plasma process since they are more directly related to fundamental plasma parameters.
It is known to use an RF sensor to monitor and control RF plasmas by measuring the Fourier components of voltage and current. The sensor can be used in closed or open loop control, as for example, in etch end-point control or as in-situ monitoring of the plasma process. In either case the plasma can be terminated when one or more of the RF Fourier components reaches predetermined limits.
Unfortunately, when problems occur in the plasma reactor due to undesired changes in input parameters, such as changes in gas flow rate, the RF sensor as previously described can only determine that a fault has occurred, i.e., it cannot determine which input parameter(s) have changed.
There is therefore a need for a method which overcomes the limitations of present fault detection schemes on plasma reactors.
Accordingly, the present invention provides a method of fault identification in a plasma process powered by an RF source, comprising the steps of:
(a) determining, in respect of a given baseline plasma process, the changes in magnitude of a plurality of Fourier components of the RF source resulting from changes in a plurality of the process input parameters from their baseline values,
(b) storing said magnitude changes as reference data,
(c) running the plasma process during a subsequent production run,
(d) during said production run, determining if there is a fault in the plasma process,
(e) if there is a fault, repeating the baseline process with input parameter values nominally the same as the baseline values of step (a),
(f) determining the magnitudes of the said Fourier components in respect of the repeat baseline process, and
(g) comparing the data determined at step (f) with the reference data to determine which input parameter(s) have changed.
In the present specification a baseline plasma process is a plasma process with pre-determined values for the process input parameters.
The invention provides a technique which allows an operator to diagnose the cause of a fault, or at least eliminate a large number of probable causes, which would otherwise prove costly to investigate.